Rotating equipment prime movers are devices that may convert one form of available energy into a particular source of continuous rotational power or motion. The rotational power produced by the prime mover is then put to use by driving another energy conversion device. Examples of rotating equipment prime movers include turbines, such as for wind, gas, steam, or water, reciprocating engines, such as for gasoline, diesel, or natural gas, and electric motors, both ac and dc, that are used to turn drive trains, generators, pumps, or compressors.
Such rotating equipment prime movers are usually equipped with a governor, a rotational velocity i.e. speed limiting and control device, to manipulate the rotational speed and preferably keep the output of the prime mover at a constant level. The governor is provided with feedback from the prime mover, which may include energy output, rotational speed, angular torque or other parameters.
One particular way of measuring the rotational speed of the prime mover includes a reference gear having a specific number of teeth and a sensor that detects the teeth passing by and delivers a pulse signal for each detection. The rotational speed may then be determined in terms of frequency by dividing the number of pulses by the measurement interval, the time interval during which a number of pulses is detected:
      F    ⁢                  [    Hz    ]    =            X      pulses              T      interval      
The accuracy of this approach mainly depends on the accuracy of determining the time duration of the measurement interval. The length of the measurement interval influences the accuracy of the measurement, but also the delay time of feedback to the governor. A longer interval increases accuracy, but increases the delay time for feedback. For an engine running at low speed a long measurement interval is required to get a valid measurement, whereas for engines running at high speed a short delay time for feedback is required to control the engine at a constant speed.
A further problem is that for certain engines, such as reciprocating engines, the angular velocity is not constant, e.g. with a stroke of the engine the angular velocity will first accelerate and decelerate thereafter.
Some prior art solutions address this by adapting the manner of calculation to the speed of the engine. For example, by introducing a threshold to select between calculating the speed based on one pulse-to-pulse delay or calculating the speed by averaging over multiple pulse-to-pulse delays. Though this allows to increase accuracy at high speed, it may show variations between consecutive calculations due to the variations in angular velocity. Other prior art solutions therefore measure the speed with an increased measurement interval, despite that this delays the feedback to the governor.